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	<h1 id="firstHeading" class="firstHeading" lang="en">Horizontal control points</h1>
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		<div id="siteSub" class="noprint">From PanoTools.org Wiki</div>
		
		
		
		
		
		
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The panorama tools <a href="PTOptimizer.html" title="PTOptimizer">optimizer</a> understands various different schemes for
aligning photos using <a href="Control_points.html" title="Control points">control points</a>:
Normal points are <b>t0</b> points, horizontal control points are <b>t1</b> points,
<a href="Vertical_control_points.html" title="Vertical control points">vertical control points</a> are <b>t2</b> points,
and <a href="Straight_line_control_points.html" title="Straight line control points">straight line control points</a> are <b>t3</b>, <b>t4</b>, etc... points.
</p><p>The main use of <b>horizontal control points</b> is to change the perspective of the output panorama such that the marked structures are horizontal in the output projection<a class="external" href="https://wiki.panotools.org/Projection">[*]</a>. In a multi image panorama this is only possible if the <a href="Optimization.html" title="Optimization">optimization</a> of <a href="Roll.html" title="Roll">Roll</a> and <a href="Pitch.html" title="Pitch">Pitch</a> is allowed for all images and of <a href="Yaw.html" title="Yaw">Yaw</a> for all but the anchor image.
</p><p>You set <b>horizontal control points</b> on a horizontal structure apart from each other. However, be careful in a <a href="Cylindrical.html" class="mw-redirect" title="Cylindrical">cylindrical</a> or <a href="Equirectangular.html" class="mw-redirect" title="Equirectangular">equirectangular</a> panorama not to set them 180° apart, since this won't level the horizon.
</p><p>Please note that all real world horizontal lines only stay horizontal in rectilinear<a class="external" href="https://wiki.panotools.org/Rectilinear">[*]</a> projection. In <a href="Cylindrical.html" class="mw-redirect" title="Cylindrical">cylindrical</a>, <a href="Equirectangular.html" class="mw-redirect" title="Equirectangular">equirectangular</a> and <a href="Fisheye.html" class="mw-redirect" title="Fisheye">fisheye</a> projections only the horizon itself stays horizontal. 
</p><p>More details in <a href="Perspective_correction.html" title="Perspective correction">Perspective correction</a> and Panotools internals<a class="external" href="https://wiki.panotools.org/Panotools_internals#Line_control_points">[*]</a>
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